This invention is concerned with mapping a surface of a workpiece, e.g. to measure the roughness of an area of the surface of a workpiece. A knowledge of such roughness may be required in testing whether a workpiece has been manufactured correctly or in studying the effects of wear on a component.
Present methods of mapping involve the use of a height measuring device in the form of a stylus mounted on a pivoted arm. The stylus rests on the surface and the angle of the arm about its pivot point indicates the height of the particular point on which the stylus rests relative to a datum level. A linear motor is used to draw the stylus along a line within said area and an electrical signal representing the path followed by the stylus is recorded. This signal is an analogue of the heights of the surface along said line. While this analogue gives a reasonable indication of the roughness of the surface along said line, it gives no information about the roughness of the surface at right angles to said line and so can give a false impression of a surface. Information, which is hereinafter referred to as "three-dimensional information", about the height variation of a surface over an area rather than along a line, is necessary where it is important that a surface has equal roughness in all directions or where a specified differential roughness in different directions is necessary.
By present methods, three-dimensional information is gathered by utilising the linear motor to draw the stylus along a plurality of parallel lines with relative motion of the component and the stylus perpendicular to said lines being caused between each operation of the linear motor. The plurality of analogue signals collected can then be processed to give a three-dimensional "picture" of the surface indicating its roughness over the area. Such a method is described in GB 2,009,409A which concerns an instrument which comprises a probe, means for oscillating the probe between a null position and contact with a workpiece, and means for moving the workpiece continuously to cause the probe to track across the workpiece surface. These methods are, however, limited in their accuracy by the inaccuracy of linear motors which cause an uncertainty in the beginning and end points of each line and a corresponding uncertainty in which points on the parallel lines are opposite one another. Furthermore, each point on a line is visited only once by a moving stylus which causes inaccuracy in the height indications, for example because the stylus may bounce. Furthermore, because of uncertainty in the position of the line investigated, such apparatus is unsuitable for investigating a surface before and after treatment or wear to investigate the effect of the treatment or the extent of the wear.
It is an object of the present invention to provide an improved method of mapping which provides better three-dimensional information.